1. Field of the invention
The present invention relates to a method of preparing a polymer for rotational molding and to a polymer particle prepared by said method.
2. Prior Art Statement
Rotational molding is used to prepare some hollow-molded articles such as storage tanks, gasoline tanks for automobiles, and some toys. Rotational molded hollow polymeric articles are prepared by placing a powdered polymeric material in a mold which is rotated on two axes (as in a martini shaker), and heating the polymeric material to its molding temperature. The polymeric material melts and coats the surface of the mold, producing a hollow article molded in the shape of the mold. Depending on the purpose for which the hollow article is to be used, the polymeric powder used to make the article can be either a thermoplastic or a thermo-set material. In those applications where a thermoset (cross-linked) polymer is used, the polymer is cured due to heating during the rotational molding operation.
In prior art methods, the polymeric powder used in cross-link rotational molding is prepared by adding raw polymer feedstock, together with a curing agent (usually a peroxide), into a melt extruder. The curing agent is incorporated into the polymer during melt extrusion thereof. The extruded material is made into pellets by conventional means. The pellets, which have the curing agent incorporated therein, are ground and screened to produce a powder having a size, about 30 to 35 mesh, suitable for rotational molding.
Since curing agents (e.g. peroxides) are very reactive, and their half-life is reduced when heated, the polymer powder produced by the prior art method (which is heated in the melt extruder and is heated a second time from frictional forces in the grinding step) contains a curing agent which has a reduced activity. Also, since some of the peroxide may be activated in the prior art method for preparing a powder for rotational molding, crosslinking of the polymer may occur prematurely.
It is known in the art to prepare, in a batch process, polymer granules, which are to be used in the preparation of insulating wire, by absorbing an organic peroxide curing agent onto the surface of said polyme granules, as is disclosed by Sumitomo Chemical Ltd. in Japanese Patent No. 57-149342.
By using the method of the present invention, intermediate melt processing steps, which are normally required to add curatives to a polymer which is being prepared for rotational molding, are not required. The possibility of premature activation of the curing agent and/or crosslinking during such intermediate steps can be substantially eliminated by use of the present invention. The present invention has the advantages that the curative, with or without an initiator, can be added to and absorbed by a polymer which is already in a form which can subsequently be used in rotational molding, i.e. in powder form, and the temperatures in the method steps of the invention can be easily controlled.